1. Field of the Invention
This invention relates to a wavefront sensor for reducing optical interference. More particularly, this invention relates to a multi-lens array design of a wavefront sensor that can reduce optical interference. The optical interference occurs because each of a plurality of focal spots formed by the multi-lens array causes a rippling effect which propagates to and interferes with an accurate measurement of neighboring focal spots.
2. Description of the Related Art
Wavefront sensors have been used, for example, in camera focusing technology to measure the distance between an object and a camera by sending out a signal wavefront and to measure the round trip time of such signal wavefront. Knowing the distance, the camera can focus properly. Particularly, wavefront sensors include elements which provide information about phase distortions or aberrations in a received wavefront, and elements which analyze, measure, and provide information signals to correct for the aberrations received in optical wavefronts.
In the laser art, wavefront sensors, such as Shack-Hartman sensors, have been used to measure the phase front quality of an incoming laser beam. The wavefront of incoming beam is defined as a surface that is normal to the local propagation direction of the beam. Wave-aberration polynomial represents the departure of the actual wavefront from a perfect spherical reference surface.
FIG. 1 schematically illustrates a conventional Shack-Hartman sensor 100 including an array of lenses 140, commonly referred to in the micro-optics technology as a multi-lens array (abbreviated to M.L.A.), to focus an incoming beam 120 to form a set of focal spots 150 (shown in FIG. 2) on a detector 130, such as a charge coupled device (CCD) camera. Detector 130 detects focal spots 150 and transmits an output to a measuring unit (not shown). The measuring unit compares the light intensity of various focal spots 150 on detector 130 with a reference beam or with a set of nominal values. Based on the readings of the measuring unit, an adaptive optical system (not shown) then compensates for errors or deviations within the laser or resulting from the atmosphere through which the laser beam travels.
A multi-lens array 140 of conventional sensor 100 may be composed of a plurality of lenses, each having a square or rectangular aperture, arranged in a two-dimension configuration. As shown in FIG. 2, focal spots 150 are formed by multi-lens array 140, each lens having a square configuration, such as illustrated by reference element 144. The distribution of focal spots 150 creates a diffraction pattern spreading widely in both the x and y directions, a square array of 6xc3x976 is shown, and are captured by CCD camera 130. For each lens of multi-lens array 140, a collection area of 9xc3x979 CCD pixels or less, illustrated by reference element 132, is assigned to capture the intensity of the central lobe of the corresponding focal spot 150.
The conventional wavefront sensor 100 often encounters a cross talk problem, i.e., a measurement error in determining the characteristics of the plurality of focal spots. FIGS. 3A and 3B schematically show the cross talk problem. The error occurs because each focal spot 150, for example, focal spot F1, radiates an energy, represented by a wave curve W1, that propagates to and directly interferes with the neighboring focal spots 150, including, for example, focal spot F2, and vice versa, energy wave W2 radiated by focal spot F2 propagates and directly interferes with focal spot F1 and other surrounding focal spots.
When multi-lens array 140 have a square or rectangular aperture configuration, such as schematically shown in FIGS. 2 and 4, focal spots F1 and F2 propagate energy waves W1 and W2, respectively, orthogonally along the x and y axes. The rippling effect of energy wave W2 along the x axis directly interferes with the measurement of intensity level of focal spot F1, and vice versa, the rippling effect of energy wave W1 directly interferes with the measurement of intensity level of focal spot F2. It can be seen that the rippling effects of energy waves W1 and W2 along the y axis also directly interfere with other neighboring focal spots 150.
In light of the foregoing, there is a need for a wavefront sensor which can eliminate or substantially reduce the cross talk problem. Also, the wavefront sensor needs to have a compact design and be insensitive to external disturbances. In addition, it is preferable that the wavefront sensor can be easily manufactured from a conventional lithography system to make the new detector and the multi-lens array.
The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and consistent with the principles of the invention, as embodied and broadly described herein, a first aspect of the invention is a method for reducing an optical interference in a wavefront sensor. The method comprises the steps of providing a multi-lens array in a two-dimension configuration to focus an incoming wavefront to form a plurality of focal spots, and systematically off-setting portions of the plurality of focal spots to create a staggered two-dimension diffraction pattern.
Another aspect of the present invention is a method for making a wavefront sensor to reduce an optical interference of focal spots. The method comprises the steps of providing a multi-lens array to focus an incoming wavefront to form a plurality of focal spots in a two-dimension configuration, and off-setting predetermined portions of the plurality of focal spots to form a staggered two-dimension diffraction pattern. The method also comprises the step of providing a detector to detect the staggered two-dimension diffraction pattern.
A further aspect of the present invention is a wavefront sensing apparatus, comprising a multi-lens array to focus an incoming wavefront to a plurality of focal spots, the multi-lens array configured to form a staggered two-dimension diffraction pattern to substantially eliminate optical interference, and a detector to detect the staggered two-dimension diffraction pattern.
Yet a further aspect of the present invention is a wavefront sensing apparatus having a multi-lens array to focus an incoming wavefront to form a plurality of focal spots, and a detector to detect the plurality of focal spots. The apparatus comprises a plurality of optical prisms attached to predetermined portions of the multi-lens array, so that corresponding predetermined portions of the plurality of focal spots are systematically off-set to form a staggered diffraction pattern on the detector.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. Additional advantages will be set forth in the description which follows, and in part will be understood from the description, or may be learned by practice of the invention. The advantages and purposes may be obtained by means of the combinations set forth in the attached claims.